Cell electrode and electrochemical cell therewith

ABSTRACT

This invention provides an electrode for an electrochemical cell in which an active material in an electrode material is a proton-conducting compound, wherein the electrode material comprises a nitrogen-containing heterocyclic compound or a polymer having a unit containing a nitrogen-containing heterocyclic moiety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an electrode used in an electrochemical cellsuch as a secondary battery and an electric double-layer capacitor andan electrochemical cell using the electrode. In particular, it relatesto an electrode having improved cycle properties without reduction in anappearance capacity, and an electrochemical cell using the electrode.

2. Description of the Related Art

There have been suggested and practically used electrochemical cells(hereinafter, referred to as “cell”) such as secondary batteries andelectric double-layer capacitors in which a proton-conducting compoundis used as an electrode active material. Such a cell is illustrated in across-sectional view of FIG. 1.

Specifically, FIG. 1 shows a cell where a positive electrode 2containing a proton-conducting compound as an active material is formedon a positive current collector 1 while a negative electrode 3 is formedon a negative current collector 4, and these electrodes are combined viaa separator 5 and where only protons are involved in an electrodereaction as a charge carrier. Also, the cell is filled with an aqueousor non-aqueous solution containing a proton source as an electrolyticsolution, and is sealed by a gasket 6.

The electrodes 2, 3 are formed as follows. A powdery doped or undopedproton-conducting compound is blended with a conductive auxiliary and abinder to prepare a slurry, which is then placed in a mold and molded bya hot press to form an electrode having a desired electrode density anda desired film thickness. Alternatively, the slurry is screen-printed ona conductive base-material and dried to form an electrode. Then, apositive electrode and a negative electrode thus formed are mutuallyfaced via a separator to give a cell.

Examples of a proton-conducting compound used as an electrode activematerial include π-conjugated polymers such as polyaniline,polythiophene, polypyrrole, polyacetylene, poly-p-phenylene,polyphenylene-vinylene, polyperinaphthalene, polyfuran, polyflurane,polythienylene, polypyridinediyl, polyisothianaphthene, polyquinoxaline,polypyridine, polypyrimidine, polyindole, polyaminoanthraquinone andtheir derivatives; indole-based compounds such as indole trimer; andhydroxyl-containing polymers such as polyanthraquinone andpolybenzoquinone where a quinone oxygen is converted into a hydroxylgroup by conjugation). These compounds may be doped to form a redox pairexhibiting conductivity. These compounds are appropriately selected as apositive active material and a negative active material, taking a redoxpotential difference into account.

Known electrolytic solutions include an aqueous electrolytic solutionconsisting of an aqueous acid solution and a non-aqueous electrolyticsolution based on an organic solvent. When using a proton-conductingcompound, the former aqueous electrolytic solution is preferentiallyused because it can give a high-capacity cell. The acid used may be anorganic or inorganic acid; for example, inorganic acids such as sulfuricacid, nitric acid, hydrochloric acid, phosphoric acid, tetrafluoroboricacid, hexafluorophosphoric acid and hexafluorosilicic acid and organicacids such as saturated monocarboxylic acids, aliphatic carboxylicacids, oxycarboxylic acids, p-toluenesulfonic acid, polyvinylsulfonicacid and lauric acid.

A cell using such a proton-conducting compound as an electrode activematerial has a short cycle life due to increase in an internalresistance, and the tendency becomes more prominent as a temperature iselevated. Furthermore, it has a drawback of insufficient long termstability under a high temperature atmosphere.

These problems are caused by aggravated deterioration atmosphere due todeceleration of proton adsorption-desorption reaction as acharge/discharge mechanism of an electrode active material. Inparticular, at an elevated temperature, peroxidation of a material ismuch more accelerated, resulting in accelerated deterioration.

An electrode active material is susceptible to deterioration in anoxidized state. It is probably because a proton (H⁺)adsorption-desorption reaction for the active material is deterioratedover time in the charge/discharge mechanism as described below. Suchdeterioration proceeds because doping/dedoping activity of the activematerial is reduced under an excess proton atmosphere rather than anoptimal proton atmosphere which depends on the identity of the activematerial and the number of reaction electrons, in a protonadsorption-desorption reaction between the active material and anelectrolyte. Thus, charge/discharge power of the cell is deteriorated.It is called “peroxidation-perreduction deterioration”; specifically,peroxidation deterioration for an active material of positive electrodeand perreduction deterioration for an active material of negativeelectrode.

This phenomenon will be described for a case where an active material ofpositive electrode is an indole derivative (indole trimer) while anactive material of negative electrode is a quinoxaline polymer. Herein,charge/discharge mechanisms for a positive and a negative electrodematerials are as indicated in chemical formulas (8) and (9),respectively, wherein R represent appropriate substituents and X⁻represents an anion.

Under a high-level acid atmosphere (low pH), the phenomenon particularlytends to occur so that deterioration in cycle properties is accelerated.For polyphenylquinoxaline which can be used as a material of negativeelectrode, tetraprotonation may be caused whereas a normal doped stateis represented by a diprotonated derivative in a charge/dischargemechanism. Thus, the active material is dissolved, leading to reductionin a charge/discharge power. An excessively higher electrolyteconcentration (proton concentration) may further accelerate oxidationdeterioration.

FIG. 6 is a graph showing variation in cycle properties to anelectrolyte concentration (sulfuric acid concentration). As seen in thisgraph, as an electrolytic solution concentration increases, a capacitydecreases according to the cycle number so that cycle properties aredeteriorated. In addition, under a low concentration atmosphere, cycleproperties are improved while an appearance capacity tends to bereduced. FIG. 7 is a graph illustrating variation in an appearancecapacity to an electrolyte concentration (sulfuric acid concentration).As seen in this graph, as an electrolyte concentration is reduced, anappearance capacity is reduced.

Electrolytic solutions comprising a nitrogen-containing heterocycliccompound as a non-aqueous electrolytic solution in the prior art havebeen described in Japanese Laid-open Patent Publication Nos. 2000-156329(Prior art 1) and 2001-143748 (Prior art 2). Japanese Laid-open PatentPublication No. 7-320780 (Prior art 3) has described a solid-electrolytesecondary battery comprising a polymer gel electrolyte consisting of,for example, an aprotic solvent and polyimidazole. Japanese Laid-openPatent Publication No. 10-321232 (Prior art 4) has described anelectrode comprising a benzimidazole derivative although an electrolyticsolution used therein is different from that in this invention.

In Prior art 1, there has been disclosed an electrolytic solution for analuminum electrolysis capacitor comprising a quaternary salt having of aquaternary cation from a compound containing N,N,N′-substituted amidinegroup and an organic acid anion, and an organic solvent. There has beendescribed that although a conventional electrolytic solution comprisinga quaternary ammonium carboxylate has a drawback that degradation of arubber packing is accelerated so that sealing performance issignificantly deteriorated, an additive having a cationic, quaternaryamidine group may improve thermal stability of the electrolytic solutionand a specific conductivity, and that in particular, a compound in whichelectrons in the amidine group are delocalized and a cation isstabilized by resonance gives an improved specific conductivity becauseof accelerated ion dissociation. There has been further described thatwhen excess hydroxide ions are generated after electrolysis in theelectrolytic solution, the hydroxide ions may rapidly disappear byreaction of the hydroxide ions and the amidine group so that unlike aconventional quaternary ammonium salt, effects of the electrolysis canbe reduced and thus degradation of a packing in a capacitor can beminimized, resulting in improved sealing performance.

Prior art 2 has disclosed an electrolytic solution for a non-aqueouselectrolyte lithium secondary battery, comprising a lithium salt of aperfluoroalkylsulfonic acid dissolved in an organic solvent and at leastone selected from heterocyclic compounds containing at least onefluorine atom and a nitrogen or oxygen atom. According to Prior art 2,the heterocyclic compound added to the electrolytic solution can form astrongly adsorptive and antioxidative film on a positive currentcollector, resulting in preventing oxidation deterioration of thepositive current collector and thus improvement in cycle properties.

Prior art 3 has disclosed a solid electrolyte secondary batterycomprising a positive electrode, a negative electrode containing lithiumas an active material, and a polymer solid electrolyte consisting of acomplex of an electrolyte salt with a polymer or a polymer gelelectrolyte prepared by impregnating an electrolytic solution of anelectrolyte salt dissolved in an aprotic solvent into a polymer, whereinthe polymer is selected from the group consisting of a polyamide,polyimidazole, a polyimide, polyoxazole, polytetrafluoroethylene,polymelamineformamide, a polycarbonate and polypropylene. There isdescribed that cycle properties are improved because the electrolyte isunreactive to the negative electrode and thus an internal resistance isunlikely to be increased even after repeating charge/discharge cycles.

For solving the problems of a reduced appearance capacity anddeteriorated cycle properties seen in FIGS. 6 and 7, it is necessary toprovide an optimal electrolyte composition (H⁺, X⁻), or to improve anelectrode for preventing peroxidation-perreduction deterioration of anelectrode active material in the reaction between an electrolyte and theactive material.

In both Prior arts 1 and 2, a nitrogen-containing heterocyclic compoundis added to a non-aqueous electrolytic solution. In Prior art 3, apolymer gel electrolyte consisting of, for example, an aprotic solventand a polyimidazole is used to make the electrolyte unreactive tolithium in the negative electrode so that increase of an internalresistance can be minimized and thus cycle properties can be improved.In any of Prior arts 1, 2 and 3, a nitrogen-containing heterocycliccompound or its polymer is added to an electrolyte, which is differentfrom this invention where a particular substance is added and blended inan electrode.

Since Prior art 4 relates to a lithium battery in which an electrolyticsolution contains an organic solvent, a proton concentration is nottaken into consideration. Thus, a mechanism of proton conductivity ordeterioration as characteristics of an active material is considerablydifferent. Prior art 4 is different from this invention in which anelectrolytic solution contains a proton source and a proton-conductingcompound is used as an active material.

SUMMARY OF THE INVENTION

An objective of this invention is to improve an electrode for preventingperoxidation-perreduction deterioration of an electrode active materialand to provide a cell electrode exhibiting improved cycle properties andan electrochemical cell comprising the electrode.

This invention provides an electrode for an electrochemical cell inwhich an active material in an electrode material is a proton-conductingcompound, wherein the electrode material comprises a nitrogen-containingheterocyclic compound or a polymer having a unit containing anitrogen-containing heterocyclic moiety.

This cell electrode may be suitably used in an electrochemical cell inwhich only protons act as a charge carrier in a redox reaction in bothelectrodes associated with charge and discharge.

This invention also provides an electrochemical cell wherein the abovecell electrode according to this invention is used for at least one ofthe electrodes and both electrodes comprise a proton-conducting compoundas an active material.

This invention also relates to the above electrochemical cell whereinonly protons can act as a charge carrier in a redox reaction in bothelectrodes associated with charge and discharge. More specifically, thisinvention relates to the electrochemical cell comprising an electrolytecontaining a proton source wherein only adsorption and desorption ofprotons in the electrode active material can be involved in electrontransfer in a redox reaction in both electrodes associated with chargeand discharge.

In this invention, the nitrogen-containing heterocyclic compound may beone or more selected from the group consisting of imidazole, triazole,pyrazole, benzimidazole and their derivatives.

The above polymer having a unit containing a nitrogen-containingheterocyclic moiety may be a polymer having a unit containing abenzimidazole, benzbisimidazole or imidazole moiety.

This invention can improve cycle properties while inhibiting reductionin an appearance capacity. This is because a nitrogen-containingheterocyclic compound or polymer having a unit containing anitrogen-containing heterocyclic moiety added to an electrode interactswith protons in an electrolyte so that only a proton concentration canbe controlled without reducing a concentration of anions acting asdopant in an adsorption-desorption reaction between the active materialand protons in the electrolyte. It is also because an optimalproton-concentration atmosphere for the reaction can be created,resulting in inhibition of deterioration due to peroxidation.

The polymer in this invention implies the compound having two or morerecurring unit, or includes so-called oligomers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of an electrochemical cell according to anembodiment of this invention.

FIG. 2 is a graph showing CV measurement results for a positiveelectrode in an aqueous sulfuric acid solution using electrodesaccording to this invention and according to the prior art.

FIG. 3 is a graph showing CV measurement results for a negativeelectrode in an aqueous sulfuric acid solution using electrodesaccording to this invention and according to the prior art.

FIG. 4 is a graph showing variation in cycle properties for batteriesaccording to this invention (Examples 1, 3, 5, 7, 14 and 19 andaccording to the prior art (Comparative Examples 1 and 2).

FIG. 5 is a graph showing variation in a cell internal resistance vs thecycle number for batteries according to this invention (Examples 1, 3,7, 14 and 19) and according to the prior art (Comparative Examples 1 and2).

FIG. 6 is a graph showing variation in cycle properties for differentsulfuric acid concentrations.

FIG. 7 is a graph showing variation in an appearance capacity fordifferent sulfuric acid concentrations.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of this invention will be detailed.

A cell electrode according to this invention is made of an electrodematerial comprising a proton-conducting compound as an active materialand a nitrogen-containing heterocyclic compound or a polymer having aunit containing a nitrogen-containing heterocyclic moiety. Another cellelectrode according to this invention is made of an electrode materialcomprising, as an active material, a polymer having a unit constitutinga proton-conducting polymer and a unit having a nitrogen-containingheterocyclic moiety (the both units may constitute one unit).

An electrochemical cell according to this invention employs the aboveelectrode according to this invention as at least one electrode andotherwise may be as with a conventional cell. An electrochemical cellaccording to this invention is preferably that wherein only protons actas a charge carrier in a redox reaction associated with charge anddischarge in both electrodes; more specifically that comprising anelectrolyte containing a proton source wherein only adsorption anddesorption of protons in the electrode active material can be involvedin electron transfer in a redox reaction in both electrodes associatedwith charge and discharge.

An electrochemical cell may have a basic configuration as shown in, forexample, FIG. 1, where a positive electrode 2 comprising aproton-conducting compound as an active material and a negativeelectrode 3 are formed on a positive current collector 1 and a negativecurrent collector 4, respectively, and these electrodes are laminatedvia a separator 5. The cell is filled with an aqueous or non-aqueoussolution containing a proton source as an electrolytic solution and issealed by a gasket 6.

The electrodes 2, 3 can be, for example, formed as follows. A powderydoped or undoped proton-conducting compound is blended with a conductiveauxiliary, a binder and a nitrogen-containing heterocyclic compound or apolymer having a unit containing a nitrogen-containing heterocyclicmoiety to prepare a slurry, which is then placed in a mold with adesired size and molded by a hot press to form an electrode having adesired electrode density and a desired film thickness. Then, a positiveelectrode and a negative electrode thus formed are mutually faced via aseparator to give a cell.

A nitrogen-containing heterocyclic compound used in this invention maybe preferably one or more selected from the group consisting ofimidazole, triazole, pyrazole, benzimidazole and their derivatives.Specifically, the nitrogen-containing heterocyclic compound representedby chemical formulas (1) to (5) may be used.

wherein R independently represent hydrogen, alkyl having 1 to 4 carbonatoms, amino, carboxyl, nitro, phenyl, vinyl, halogen, acyl, cyano,trifluoromethyl, alkylsulfonyl and trifluoromethylthio.

Examples of a halogen atom include fluorine, chlorine, bromine andiodine. Examples of an alkyl group having 1 to 4 carbon atoms includemethyl, ethyl, propyl, isopropyl, n-butyl, s-butyl, isobutyl andt-butyl. An acyl group may be selected from those having an alkyl having1 to 4 carbon atoms as described above. An alkylsulfonyl group may beselected from those having an alkyl having 1 to 4 carbon atoms asdescribed above.

A polymer having a unit containing a nitrogen-containing heterocyclicmoiety may be a polymer having a unit containing a benzimidazole,benzbisimidazole or imidazole moiety; for example, a nitrogen-containingbasic polymer such as a benzimidazole-based polymer represented bychemical formula (6) or (10) and a polyvinylimidazole represented bychemical formula (7), polybenzbisimidazole, benzbisimidazole-basedpolymer represented by chemical formula (11) or polyimidazolerepresented by chemical formula (12).

wherein n represents a positive integer, and H bonded to N may beindependently replaced with a substituent selected from theabove-described R.

wherein n represents a positive integer, H bonded to N may beindependently replaced with a substituent selected from theabove-described R, and R1 represents a divalent group such as analkylene having 1 to 4 carbon atoms and a substituted or non-substitutedphenylene.

Using such an electrode, a reaction described below may occur with ionsin an electrolytic solution containing a proton source. When thenitrogen-containing heterocyclic compound is imidazole, imidazoleadsorbs a proton as shown in chemical formula (13).

wherein n represents a positive integer and m represents an integerlarger than n.

Such proton adsorption by imidazole results in prevention ofperoxidation or perreduction of an active material of positive and thusa longer cycle life of the cell. As described above, a concentration ofprotons involved in a reaction with an active material may beappropriately adjusted by controlling the amount of anitrogen-containing heterocyclic compound or polymer having a unitcontaining a nitrogen-containing heterocyclic moiety to be added andblended, without varying a concentration of an anion to be a dopant.Thus, a higher appearance capacity of the cell can be maintained andcycle properties can be improved.

A polymer having a unit containing a nitrogen-containing heterocyclicmoiety may be a polymer having a unit constituting a conventionalproton-conducting polymer and a unit of a nitrogen-containingheterocyclic compound or of a monomer compound having anitrogen-containing heterocyclic moiety. The polymer acts as aproton-conducting active material as well as an inhibitor ofperoxidation-perreduction deterioration for the abovenitrogen-containing heterocyclic compound. An electrode comprising thepolymer as an electrode active material can, therefore, exhibitimprovement equivalent to that achieved by an electrode comprising theabove nitrogen-containing heterocyclic compound or the polymer having aunit containing a nitrogen-containing heterocyclic moiety. In otherwords, there may be provided a cell where peroxidation-perreductiondeterioration is much more reduced in comparison with a conventionalelectrode as described later even under a high proton concentrationatmosphere.

In the light of inhibition of peroxidation-perreduction deterioration, acopolymerization composition for a polymer having a unit containing anitrogen-containing heterocyclic moiety according to this invention maybe such that a unit containing a nitrogen-containing heterocyclic moietyis preferably at least 5 mol %, more preferably at least 10 mol %. Onthe other hand, in the light of its function as an active material suchas a capacity appearance rate, the unit may be contained in an amount ofpreferably 90 mol % or less, more preferably 80 mol % or less. Thepolymer having the weight-average molecular weight of 1000 to 50000,preferably 3000 to 15000 measured with GPC may be used in thisinvention.

For determining the effects of this invention, a positive electrode(comprising an indole trimer as an active material) was evaluated bycyclic voltammetry (CV-measurement). In this measurement, a workingelectrode was an electrode formed by depositing a mixture of an activematerial of positive electrode with imidazole on a carbon sheet; acounter electrode was a platinum electrode; and a reference electrodewas an Ag/AgCl electrode. A measuring temperature was 25° C., a scanvoltage ranged from 600 to 1100 mV, and a scan speed was 1 mV/sec. Anelectrolytic solution was a 20 wt % aqueous solution of sulfuric acid,and a composition of a positive electrode material as the workingelectrode was that described in Example 3 (containing 20 wt % ofimidazole). An electrode without imidazole (Comparative Example 1described later) was also evaluated as a reference example. The resultsare shown in a graph in FIG. 2.

The results show that reduction in a discharge capacity in Example 3 isless than that in Comparative Example 1. In relation to ComparativeExample 1, a redox potential in Example 3 was shifted to lower potentialside by several tens of mV. That is, shift to a stable potential atwhich oxidation deterioration is reduced was observed. It may beconcluded that a cycle life was prolonged.

An active material of negative electrode (polyphenylquinoxaline) wasalso evaluated by CV measurement using the negative electrodes describedin Example 3 and Comparative Example 1. FIG. 3 shows the results ofvariation in their discharge capacity. The results show thatdeterioration in a capacity due to excessive protonation of the activematerial of negative electrode was inhibited.

It was, therefore, shown that this invention can prevent deteriorationin both electrodes, a positive electrode and a negative electrode.

In the above examples, an aqueous electrolytic solution has beendescribed. However, in this invention, an electrolyte may be anyelectrolyte containing a proton source, and reduction in a capacity maybe similarly inhibited for a different type of electrolyte such as anon-aqueous electrolytic solution, a gel electrolyte and a solidelectrolyte. In both cases using a cell electrode comprising the abovenitrogen-containing heterocyclic compound and using a cell electrodecomprising the polymer having a unit containing a nitrogen-containingheterocyclic moiety, inhibition in capacity reduction (inhibition ofactive material deterioration) can be obtained.

An electrode active material constituting a cell electrode of thisinvention exhibits conductivity by being doped to form a redox pair, andthus may be a proton-conducting compound known in the art. Aproton-conducting compound means a compound which can generate anelectrochemical reaction involving only adsorption and desorption ofprotons in electron transfer associated with a redox reaction. Examplesof such a compound include n-conjugated polymers such as polyanilinepolymers (e.g., polyaniline), polythiophene, polypyrrole, polyacetylene,poly-p-phenylene, polyphenylene-vinylene, polyperinaphthalene,polyfuran, polyflurane, polythienylene, polypyridinediyl,polyisothianaphthene, polyquinoxaline, polypyridine, polypyrimidine,polyindole, polyaminoanthraquinone and their derivatives; indole-basedcompounds such as indole trimer and its derivative; andhydroxyl-containing polymers such as polyanthraquinone andpolybenzoquinone where a quinone oxygen is converted into a hydroxylgroup by conjugation. These compounds are appropriately selected as anactive material of positive electrode or negative electrode, taking aredox potential difference into account.

Among these, an active material of positive electrode is preferablyselected from the group consisting of polyaniline, polydianiline,polydiaminoanthraquinone, polybiphenylaniline, polynaphthylaniline,polyindole and indole-based compounds. An active material of negativeelectrode is preferably selected from the group consisting ofpolypyridine, polypyrimidine, polyquinoxaline and their derivatives. Inparticular, preferred is a combination of an indole-based compound as anactive material of positive electrode and a quinoxaline-based polymer asan active material of negative electrode. The indole-based compound ispreferably one or more of an indole trimer and its derivatives (anindole trimer compound) while the quinoxaline-based polymer ispreferably polyphenylquinoxaline.

An indole trimer compound has a fused polycyclic structure comprising asix-membered ring formed by atoms at the second and the third positionsin three indole rings. The indole trimer compound can be prepared fromone or more compounds selected from indole or indole derivatives oralternatively indoline or its derivatives, by a known electrochemical orchemical process.

Examples of such indole trimer compound include those represented by thefollowing chemical formulas:

wherein R independently represent hydrogen, halogen, hydroxyl, carboxyl,sulfone, sulfate, nitro, cyano, alkyl, aryl, alkoxyl, amino, alkylthioor arylthio.

In these formulas, examples of halogen in R include fluorine, chlorine,bromine and iodine. Examples of alkyl in R in these formulas includemethyl, ethyl, propyl, isopropyl, n-butyl, s-butyl, isobutyl, t-butyl,n-pentyl, n-hexyl, n-heptyl and n-octyl. Alkoxy in R in these formulasis a substituent represented by —OX, wherein X may be alkyl as describedabove. Examples of aryl in R in these formulas include phenyl, naphthyland anthryl. The alkyl moiety in alkylthio in R in these formulas may beselected from those described above. The aryl moiety in arylthio in R inthese formulas may be selected from those described above.

A quinoxaline-based polymer is a polymer having a unit containing aquinoxaline moiety which may be represented by any of the followingformulas (16) and (17). A preferable quinoxaline-based polymer is apolymer having a unit containing 2,2-(p-phenylene)diquinoxaline moietyrepresented by the formula (17).

wherein n represents a positive integer.

An electrolyte in this invention may be any electrolyte containing aproton source, preferably an electrolytic solution containing a protonsource, particularly an aqueous solution of sulfuric acid. A protonsource may be an inorganic or organic acid. Examples of an inorganicacid include sulfuric acid, nitric acid, hydrochloric acid, phosphoricacid, tetrafluoroboric acid, hexafluorophosphoric acid andhexafluorosilicic acid. Examples of an organic acid include saturatedmonocarboxylic acids, aliphatic carboxylic acids, oxycarboxylic acids,p-toluenesulfonic acid, polyvinylsulfonic acid and lauric acid.

A proton concentration in an electrolytic solution containing a protonsource is preferably 10⁻³ mol/L or more, more preferably 10⁻¹ mol/L ormore in the light of reactivity of the electrode materials while beingpreferably 18 mol/L or less, more preferably 7 mol/L or less in thelight of deterioration in activity of the electrode materials andprevention of dissolution.

A content of a nitrogen-containing heterocyclic compound or a polymerhaving a unit containing a nitrogen-containing heterocyclic moiety in acell electrode may be appropriately selected depending on the type ofthe compound or polymer and the type and a concentration of theelectrolyte. If it is too low, oxidation deterioration of an activematerial may be inadequately inhibited. If the content is too high, anappearance capacity may be reduced, leading to deterioration in otherproperties. The content is, therefore, preferably 1 to 80 parts byweight to 100 parts by weight of the active material.

EXAMPLES

This invention will be described with reference to, but not limited to,examples, and variations may be acceptable in this invention withoutdeparting from the gist of this invention. There will be describedexamples of application to a secondary battery, but this invention maybe suitably applied to another electrochemical cell such as an electricdouble layer capacitor by properly adjusting parameters such as acapacity and a charge/discharge rate.

Example 1

A positive electrode used was prepared as follows. To indole trimer 69wt % as an active material were added 23 wt % of vapor growth carbon(VGCF) as a conductive auxiliary and 8 wt % of a polyfluorovinylidene(average molecular weight: 1100) as an electrode molding component. To100 wt % of the mixture was added 5 wt % of imidazole. The resultantmixture was stirred and blended in a blender and then molded by a hotpress into a solid electrode having a desired size, which was used as apositive electrode 2.

A negative electrode used was prepared as follows. Topolyphenylquinoxaline 75 wt % as an active material were added 25 wt %of carbon black (K.B.600) as a conductive auxiliary. To 100 wt % of themixture was then added 5 wt % of imidazole. The resultant mixture wasstirred and blended in a blender and then molded by a hot press into asolid electrode having a desired size, which was used as a negativeelectrode 3.

An electrolytic solution used was a 20 wt % aqueous solution of sulfuricacid.

A separator 5 used was a cation-exchange membrane with a thickness of 10to 50 μm.

The positive electrode and the negative electrode were laminatedtogether via a separator such that their electrode surfaces mutuallyfaced, and a gasket was mounted to form a battery as shown in FIG. 1.

Example 2

A positive electrode was prepared as described in Example 1 withoutadding imidazole. A negative electrode was prepared as described inExample 1 except adding 20 wt % of imidazole. A battery was formed asdescribed in Example 1, except these electrodes were used.

Example 3

A positive electrode was prepared as described in Example 1 exceptadding 20 wt % of imidazole. A negative electrode was prepared asdescribed in Example 1 except adding 20 wt % of imidazole. A battery wasformed as described in Example 1, except these electrodes were used.

Example 4

A positive electrode was prepared as described in Example 1 exceptadding 50 wt % of imidazole. A negative electrode was prepared asdescribed in Example 1 except adding 50 wt % of imidazole. A battery wasformed as described in Example 1, except these electrodes were used.

Example 5

A positive electrode was prepared as described in Example 1 exceptadding 20 wt % of imidazole. A negative electrode was prepared asdescribed in Example 1 except adding 20 wt % of 1,2,4-triazole. Abattery was formed as described in Example 1, except these electrodeswere used.

Example 6

A positive electrode was prepared as described in Example 1 exceptadding 20 wt % of 2-phenylimidazole instead of the imidazole. A negativeelectrode was prepared as described in Example 1 except adding 20 wt %of 2-phenylimidazole. A battery was formed as described in Example 1,except these electrodes were used.

Example 7

A positive electrode was prepared as described in Example 1 exceptadding 20 wt % of 3-trifluoromethylbenzimidazole instead of theimidazole. A negative electrode was prepared as described in Example 1except adding 20 wt % of 3-trifluoromethylbenzimidazole instead of theimidazole. A battery was formed as described in Example 1, except theseelectrodes were used.

Example 8

A positive electrode was prepared as described in Example 1 exceptadding 20 wt % of imidazole. A negative electrode was prepared asdescribed in Example 1 except adding 20 wt % of3-trifluoromethylbenzimidazole instead of the imidazole. A battery wasformed as described in Example 1, except these electrodes were used.

Example 9

A positive electrode was prepared as described in Example 1 exceptadding 10 wt % of imidazole and 10 wt % of 1,2,4-triazole. A negativeelectrode was prepared as described in Example 1 except adding 20 wt %of 1,2,4-triazole instead of the imidazole. A battery was formed asdescribed in Example 1, except these electrodes were used.

Example 10

A positive electrode was prepared as described in Example 1 exceptadding 10 wt % of imidazole and 10 wt % of3-trifluoromethylbenzimidazole. A negative electrode was prepared asdescribed in Example 1 except adding 10 wt % of 1,2,4-triazole and 10 wt% of 3-trifluoromethylpyrazole instead of the imidazole. A battery wasformed as described in Example 1, except these electrodes were used.

Example 11

A positive electrode was prepared as described in Example 1 exceptadding 60 wt % of imidazole. A negative electrode was prepared asdescribed in Example 1 except adding 60 wt % of imidazole. Anelectrolytic solution used was a 30 wt % aqueous solution of sulfuricacid. A battery was formed as described in Example 1, except theseelectrodes and the electrolytic solution were used.

Example 12

A positive electrode was prepared as described in Example 1 withoutadding imidazole. A negative electrode was prepared as described inExample 1 except adding 5 wt % of polybenzimidazole instead of theimidazole. A battery was formed as described in Example 1, except theseelectrodes were used.

Example 13

A positive electrode was prepared as described in Example 1 exceptadding 5 wt % of polybenzimidazole instead of the imidazole. A negativeelectrode was prepared as described in Example 1 except adding 5 wt % ofpolybenzimidazole instead of the imidazole. A battery was formed asdescribed in Example 1, except these electrodes were used.

Example 14

A positive electrode was prepared as described in Example 1 adding 20 wt% of polybenzimidazole instead of the imidazole. A negative electrodewas prepared as described in Example 1 adding 20 wt % ofpolybenzimidazole instead of the imidazole. A battery was formed asdescribed in Example 1, except these electrodes were used.

Example 15

A positive electrode was prepared as described in Example 1 exceptadding 20 wt % of polyvinylimidazole instead of the imidazole. Anegative electrode was prepared as described in Example 1 except adding20 wt % of polyvinylimidazole instead of the imidazole. A battery wasformed as described in Example 1, except these electrodes were used.

Example 16

A positive electrode was prepared as described in Example 1 exceptadding 10 wt % of polybenzimidazole and 10 wt % of polyvinylimidazoleinstead of the imidazole. A negative electrode was prepared as describedin Example 1 except adding 20 wt % of polyvinylimidazole instead of theimidazole. A battery was formed as described in Example 1, except theseelectrodes were used.

Example 17

A positive electrode was prepared as described in Example 1 exceptadding 20 wt % of imidazole. A negative electrode was prepared asdescribed in Example 1 except adding 10 wt % of polybenzimidazole and 10wt % of polyvinylimidazole instead of the imidazole. A battery wasformed as described in Example 1, except these electrodes were used.

Example 18

A positive electrode was prepared as described in Example 1 exceptadding 20 wt % of 3-trifluoromethylpyrazole instead of the imidazole. Anegative electrode was prepared as described in Example 1 except adding10 wt % of polybenzimidazole and 10 wt % of polyvinylimidazole insteadof the imidazole. A battery was formed as described in Example 1, exceptthese electrodes were used.

Example 19

A positive electrode was prepared as described in Example 1 withoutadding imidazole. As an active material of a negative electrode, aproton-conducting polymer (Mw: 10000) having the unit represented by theformula (18) was prepared by condensation polymerization of3,3-diaminobenzidine (DABZ) and 1,4-bisbenzil (BBZ) in the presence ofterephthalaldehyde using a platinum catalyst in DMF solvent. A negativeelectrode comprising the polymer having the units containing thephenylqinoxaline moiety and the benzimidazole moiety (75 wt %) and aconductive auxiliary (25 wt %) was prepared. A battery was formed asdescribed in Example 1, except these electrodes were used.

Example 20

A positive electrode was prepared as described in Example 1 exceptadding 1,2,4-triazole. A negative electrode was prepared as described inExample 19. A battery was formed as described in Example 1, except theseelectrodes were used.

Example 21

A positive electrode was prepared as described in Example 1 exceptadding 1,2,4-triazole instead of the imidazole. A negative electrode wasprepared as described in Example 1 except adding 10 wt % (to the contentof polyphenylquinoxaline) of a proton-conducting polymer having a unitcontaining a nitrogen-containing heterocyclic moiety as described inExample 19 instead of the imidazole. A battery was formed as describedin Example 1, except these electrodes were used.

Example 22

A positive electrode was prepared as described in Example 1 withoutadding imidazole. A negative electrode was prepared as described inExample 1 except adding 10 wt % (to the content ofpolyphenylquinoxaline) of a proton-conducting polymer having a unitcontaining a nitrogen-containing heterocyclic moiety as described inExample 19 instead of the imidazole. A battery was formed as describedin Example 1, except these electrodes were used.

Example 23

A positive electrode was prepared as described in Example 1 withoutadding imidazole. A negative electrode was prepared as described inExample 1 except adding 10 wt % (to the content ofpolyphenylquinoxaline) of a proton-conducting polymer having a unitcontaining a nitrogen-containing heterocyclic moiety as described inExample 19 and 10 wt % (to the content of polyphenylquinoxaline) ofpolybenzimidazole instead of the imidazole. A battery was formed asdescribed in Example 1, except these electrodes were used.

Example 24

A positive electrode was prepared as described in Example 1 withoutadding imidazole. A negative electrode was prepared as described inExample 1 except adding 50 wt % (to the content ofpolyphenylquinoxaline) of a proton-conducting polymer having a unitcontaining a nitrogen-containing heterocyclic moiety as described inExample 19 and 10 wt % (to the content of polyphenylquinoxaline) ofpolybenzimidazole instead of the imidazole. A battery was formed asdescribed in Example 1, except these electrodes were used.

Comparative Example 1

Electrodes were prepared as described in Example 1 without addingimidazole in either electrode. A battery was formed as described inExample 1, except these electrodes were used.

Comparative Example 2

Electrodes were prepared as described in Example 1 without addingimidazole in either electrode. An electrolytic solution used was a 30 wt% aqueous solution of sulfuric acid. A battery was formed as describedin Example 1, except these electrodes and the electrolytic solution wereused.

The batteries prepared in Examples 1 to 24 and Comparative Examples 1and 2 were evaluated for an appearance capacity and cycle properties.The results are shown in Table 1. TABLE 1 Cell internal Appearanceresistance capacity Cycle properties variation (%) (%) ratio(%) Exam. 198.4 83.4 118 Exam. 2 99.9 82.6 119 Exam. 3 97.2 88.3 111 Exam. 4 85.890.5 107 Exam. 5 100.1 84.6 115 Exam. 6 99.9 85.7 115 Exam. 7 100.1 86.8115 Exam. 8 97.8 82.1 119 Exam. 9 102.3 86.7 115 Exam. 10 99.4 85.2 115Exam. 11 99.9 82.9 119 Exam. 12 100.9 85.6 114 Exam. 13 102.1 88.8 111Exam. 14 101.5 93.4 106 Exam. 15 100.6 93.2 106 Exam. 16 100.1 90.8 109Exam. 17 100.1 86.7 112 Exam. 18 99.8 86.4 113 Exam. 19 104.2 94.9 105Exam. 20 102.8 96.4 104 Exam. 21 101.3 95.2 102 Exam. 22 102.1 92.6 105Exam. 23 100.5 91.9 107 Exam. 24 101.9 94.1 105 Comp. Ex. 1 100.0 80.1121 Comp. Ex. 2 102.6 65.0 138

In Table 1, an appearance capacity is a relative value (%) calculated toan appearance capacity in Comparative Example 1 (100%). Cycle propertiesis expressed as a relative discharge capacity (%) (measured at 25° C.)to a discharge capacity at the initiation of the cycles. Cell internalresistance variation ratio is a relative value (%) of a direct-currentresistance after 10,000 cycles to a direct-current resistance at theinitiation of the cycles. Cycle conditions were as follows; charging:CCCV discharge at 1 A and 1.2 V for 10 min, discharging: CC discharge at0.2 A (equivalent to 1 C), and final voltage: 0.8 V.

FIGS. 4 and 5 show the evaluation results of Examples 1, 3, 5, 7, 14 and19 and Comparative Examples 1 and 2 for cycle properties and cellinternal resistance variation ratio. As seen from discharge capacityvariation in FIG. 4, as the cycle number increased, a discharge capacitywas reduced to 80% and 65% in Comparative Examples 1 and 2,respectively, while discharge capacities in Examples were less reducedto 83% to 96%. It indicates that a discharge capacity is less varied inExamples.

As seen from cell internal resistance variation ratio in FIG. 5, cellinternal resistance variation ratios in Examples 1, 3, 7, 14 and 19 were105 to 118%, while cell internal resistance variation ratios inComparative Examples 1 and 2 were 121% and 138%, respectively. Itindicates that cell internal resistance variation in Examples is lessthan that in Comparative Example 1 or 2.

These results show that this invention can improve cycle propertieswhile inhibiting reduction in an appearance capacity.

Although these Examples employ indole trimer or polyphenylquinoxaline asan active material, an active material is not limited to those, but anyactive material having proton conductivity may be suitably used.

1. An electrode for an electrochemical cell, which comprises: aproton-conducting compound as an active material; and a polymercomprising a unit containing a nitrogen-containing heterocyclic moiety.2. The electrode of claim 1, wherein the nitrogen-containingheterocyclic moiety is a moiety selected from the group consisting of abenzimidazole moiety, a benzbisimidazole moiety and an imidazole moiety.3. The electrode of claim 1, wherein the polymer is a polybenzimidazolerepresented by formula (6) or a polyvinylimidazole represented byformula (7);

wherein n represents a positive integer.
 4. The electrode of claim 1,wherein the polymer further comprises a unit containing aproton-conducting moiety.
 5. The electrode of claim 1, wherein a contentof the unit containing the nitrogen-containing heterocyclic moiety inthe polymer is at least 5 mol %.
 6. The electrode of claim 1, furthercomprising a nitrogen-containing heterocyclic compound.
 7. The electrodeof claim 6, wherein the nitrogen-containing heterocyclic compound is oneor more compounds selected from the group consisting of imidazole,triazole, pyrazole, benzimidazole and their derivatives.
 8. Theelectrode of claim 6, wherein the nitrogen-containing heterocycliccompound is one or more compounds selected from the group consisting ofimidazole or its derivatives represented by formula (1), trizole or itsderivatives represented by formula (2) or (3), pyrazole or itsderivatives represented by formula (4) and benzimidazole or itsderivatives represented by formula (5):

wherein R independently represents hydrogen, alkyl having 1 to 4 carbonatoms, amino, carboxyl, nitro, phenyl, vinyl, halogen, acyl, cyano,trifluoromethyl, alkylsulfonyl or trifluoromethylthio.
 9. The electrodeof claim 1, comprising 1 to 80 parts by weight of the polymer to 100parts by weight of the active material.
 10. The electrode of claim 6,comprising 1 to 80 parts by weight of the nitrogen-containingheterocyclic compound and the polymer to 100 parts by weight of theactive material.
 11. An electrochemical cell, comprising: a positiveelectrode comprising a proton-conducting compound as an active material;and a negative electrode comprising a proton-conducting compound as anactive material; wherein at least one of the electrodes is the electrodeas claimed in claim
 1. 12. An electrochemical cell of claim 11,comprising an electrolyte containing a proton source wherein onlyprotons act as a charge carrier in a redox reaction in both electrodesassociated with charge and discharge.
 13. A secondary battery comprisingthe electrochemical cell of claim
 11. 14. An electrode for anelectrochemical cell, comprising: a proton-conducting polymer comprisinga unit containing a proton-conducting moiety and a unit containing anitrogen-containing heterocyclic moiety, as an active material.
 15. Theelectrode of claim 14, wherein the nitrogen-containing heterocyclicmoiety is a moiety selected from the group consisting of: abenzimidazole moiety, a benzbisimidazole moiety, and an imidazolemoiety.
 16. The electrode of claim 14, wherein the unit containing theproton-conducting moiety is a unit selected from the group consistingof: a quinoxaline moiety and a phenylquinoxaline moiety.
 17. Theelectrode of claim 14, wherein the proton-conducting polymer is apolymer comprising a unit represented by formula (18):


18. An electrochemical cell, comprising: a first electrode containing apolymer comprising a unit containing a nitrogen-containing heterocyclicmoiety; a second electrode; and a separator separating the firstelectrode and the second electrode.
 19. The electrochemical cell ofclaim 18, wherein the nitrogen-containing heterocyclic moiety is amoiety selected from the group consisting of: a benzimidazole moiety, abenzbisimidazole moiety and an imidazole moiety.
 20. The electrochemicalcell of claim 18, wherein the first electrode further comprises aproton-conducting compound as an active material.
 21. Theelectrochemical cell of claim 18, wherein the polymer is a protonconducting polymer comprising a unit containing a proton-conductingmoiety and a unit containing a nitrogen-containing heterocyclic moiety,the polymer being an active material.
 22. The electrochemical cell ofclaim 18, wherein the second electrode comprises a proton-conductingcompound as an active material.
 23. The electrochemical cell of claim18, wherein the second electrode comprises a proton-conducting compoundas an active material, and a nitrogen-containing heterocyclic compound.24. The electrochemical cell of claim 18, wherein the second electrodecomprises a proton-conducing compound as an active material, and apolymer comprising a unit containing a nitrogen-containing heterocyclicmoiety.
 25. The electrochemical cell of claim 18 further comprising aproton-ionizing electrolyte.